There's got to be a reason why the 100-400 is only $360 more new. The 100-400 is a zoom, same aperture at 400mm and it also have IS. There's got to be a catch to it. They both are pretty sharp. So what's the reasoning behind buying the prime over the zoom? Is it build quality ,IQ, something else?

The 400mm F5.6 has been excellent on any camera body I tried it on. A excellent nature and birding lens.
sharp wide open, fast focusing, excellent contrast and colors
It has no IS, but I think that is better. The old 2 stop IS on the 100-400 is not very useful anyway.
I have no problem getting sharp image with the 400mm F5.6 hand held

I use extension tubes to allow the 400mm F5.6 to focus closer on occasion
on the 5D III the 400mm F5.6 / Tamron 1.4X non-reporting TC even works for BIF

IQ is about a wash, with a slight edge (literally) to the prime. AF of the prime is much better and it's a true 400mm or close whereas the zoom is closer to 385-90mm. The Zoom has vastly improved mfd making it ideal for close-up work and of course is just so versatile. Both take a 1.4x TC very well, and here the prime will be the best choice as it'll AF better. Prime is noticeably lighter, 1.1kg vs 1.4kg.

If you are going to be at 400mm most of the time the prime makes more sense, but if you want about the most versatile lens that pushes the prime hard on IQ the zoom is the choice.

Both are great lenses but I think the prime is sharper, has faster AF, and easier to tote around. I also think the prime handles the 1.4x converter better.

The zoom is a good lens as well and does offer the focal length versatility. I ended up using my 300mm f2.8 with converters more often and got rid of my copy. If I ever went back though, I'd get the prime.

The 400/5.6 nice as it is is a dead prime.
It offers no benefit in speed stuck at 5.6..
It has no IS a big drawback with any extra benefit in percieved sharpness being lost by a higher loss rate at 400mm.
Lack of flexibility albeit at the slight drop in IQ for the zoom but as discussed numerous times the 100-400 has great IQ for its price etc.
In short unless you need the best IQ you can buy for you dollar only at 400 or with a converter it makes more sense to buy the zoom.

gome1122 wrote:
There's got to be a reason why the 100-400 is only $360 more new. The 100-400 is a zoom, same aperture at 400mm and it also have IS. There's got to be a catch to it. They both are pretty sharp. So what's the reasoning behind buying the prime over the zoom? Is it build quality ,IQ, something else?

There are a lot of reasons for lens pricing, and the pricing isn't always directly tied to value or quality or usefulness. But rather than worrying too much about that it makes more sense to think about which if these fine lenses provides functionality more suited to what you need the lens to do.

The 400mm f/5.6 (which I do not own) is regarded as a fine lens that can provide slightly better IQ at 400mm than the zoom at 400mm. If your need is for a lens that shoots only at 400mm, there is probably no reason for you to choose the zoom over the prime. (Though there could be if space for carrying gear while traveling is an issue.) Since it does not have IS, you could find yourself a bit more limited when it comes to hand holding the camera for low light shooting. If you shoot the lens from the tripod or always shoot in excellent light the lack of IS is perhaps not a concern for you. I believe those who say that it may AF a bit faster.

The 100-400mm L zoom (which I do own) is also a very fine lens. Contrary to what you sometimes read, I get excellent image quality from this lens, even at 400mm. While I do not doubt those who say that that prime is a bit better, the zoom can do very well at 400mm, too. IS can help for handheld shooting in low light. Sometimes I'm glad I have that when photographing birds very early or late in the day, when I'm already pushing boundaries of aperture, ISO, and shutter speed. The zoom provides more flexibility, which can be very useful - when I'm shooting wildlife I can often zoom back to 100mm to photograph the larger scene (a bunch of critters rather than just one, for example) and then zoom right back to 400mm without changing lens. An additional bonus from the zoom is that it packs smaller due to the extending zoom design - which can be important when traveling.

Someone mentioned the use of 1.4x teleconverters. Both are subject to the same limitation here. Most Canon DSLRs cannot AF (without some work-arounds that most won't want to try) at apertures smaller than f/5.6... which happens to be the largest aperture that both provide at 400mm. Some Canon 1-series cameras (look them up) can AF at f/8, but unless you have one of those this issue is moot.

In the end, both are fine lenses and neither is perfect - each has its strengths and weaknesses. You can make great photographs with either, so I think it is most important to think about the functional capabilities of the lenses in context of your intended use.

My recommendation is that you buy the one which will provide more opportunities to get the shot, and that's especially true when it must be hand held. Technique will have far more influence on the IQ than the lab result. For me & anyone I know the zoom is the right choice